Sorry in advance for the wall of text:

I think the national/ethnic question is an incredibly difficult question to answer, and it will take more than simply pitting 20th century revolutionaries against each other.

100%, it's not about declaring one or the other as "correct" but understanding their positions within the broader context. What I see as a goal of criticism like this is to allow for us to understand past failures and their impact on the present as well as how to move forward, rather than trying to lionize any particular figure. They are all flawed, but still worth learning from.

But it does lead us to ask, if the Bolsheviks didn't give up their claim to inherit the Russian empire, would Rosa have just decried them as another imperialist?

Based on the nature of her critique I don't think that she would simply have changed her tune given different circumstances, if anything I think she was hoping that those national liberation slogans were just that, slogans to rally support then set aside, rather than integrated into the socialist project once the Bolsheviks won. I do want to reiterate that she was clearly in support of the Bolsheviks, despite having criticisms for them, she had far worse things to say about social democrats and reformers:

the Social Democracy in the highly developed West is made up of wretched cowards and will look calmly on while the Russians bleed.

She had criticism for the Bolsheviks from the standpoint of what she saw as the needs of the world revolution, but her primary criticism was of the failure on the part of the proletariat of Western Europe to aid them.

On the subject of the Bolsheviks signing Brest-Litovsk, which was the basis for the criticism I cited in the previous post she had this to say about the choice from a non theoretical standpoint:

they are not to blame. They are in a jam, have only the choice between two evils and choose the lesser. Others are responsible for the fact that the Russian Revolution turns out to the devil’s advantage.” And again she writes: “The German government-socialists may shout that the rule of the Bolsheviks is a caricature of the dictatorship of the proletariat. If it was or is, then only for the reason that it is a product of the conduct of the German proletariat, conduct which was a caricature of socialist class struggle.”

At the end of the day and with the benefit of hindsight the Bolsheviks did make the best of a bad situation and while it's possible to envision a better path it doesn't mean that it was going to be viable in the historical context.

I only say this because much of her theories appear to simply be monday morning quarterbacking to Bolshevik ideas that actually got to be implemented

This was contemporaneous criticism though, her polemics with Lenin happened before any of this unfolded.

While Lenin and the Bolshevik line was centralization, the national projects were ultimately in opposition to that.

Quoting Lenin now on this:

the right to self-determination is an exception to the general premise of centralisation. This exception is absolutely essential in view of reactionary Great-Russian nationalism; and any rejection of this exception is opportunism (as in the case of Rosa Luxemburg); it means foolishly playing into the hands of reactionary Great-Russian nationalism. But exception must not be too broadly interpreted. In this case, there is not and must not be anything more than the right to secede." (Letter to S.G. Shahnmyan, Vol. 19).

My read of this is that both parties have their own perspective in this situation, Rosa with her perspective with Polish bourgeois nationalists, Lenin with the understanding of what reactionary russian nationalism could lead to un-checked. Not to be a centrist but they both have valid points.

As Lenin called her an opportunist, she said the same, the slogans of national liberation was commented on as opportunism and a concession to bourgeois and petty-bourgeois classes that would ultimately be a poison pill to the entire project, this is from 1915, well before these events played out:

Capitalist politicians, in whose eyes the rulers of the people and the ruling classes are the nation, can honestly speak of the “right of national self-determination” in connection with such colonial empire. To the socialist, no nation is free whose national existence is based upon the enslavement of another people, for to him colonial peoples, too, are human beings, and, as such, parts of the national state. International socialism recognises the right of free independent nations, with equal rights. But socialism alone can create such nations, can bring self-determination of their peoples. This slogan of socialism is like all its others, not an apology for existing conditions, but a guidepost, a spur for the revolutionary, regenerative, active policy of the proletariat. So long as capitalist states exist, i.e., so long as imperialistic world policies determine and regulate the inner and the outer life of a nation, there can be no “national self-determination” either in war or in peace.

In the present imperialistic milieu there can be no wars of national self-defence. Every socialist policy that depends upon this determining historic milieu, that is willing to fix its policies in the world whirlpool from the point of view of a single nation, is built upon a foundation of sand.

after all, today we could just as easily point towards the Balkans as a proof that not giving people their own national identities eventually leads to another kind of destruction and reactionary uprising, where the proletariat are easily pitted against each other by the ethnic bourgeois under 'freedom from communist tyranny'.

That still happened in the context of an imperialist world system. Once the USSR fell their days were numbered. I would still take the lesson from that to be that as long as capitalist states exist, the system operates on that logic and no other states can have self determination. Socialist states will be besieged by counterevolution and reaction, nationalist states will be limited by their own colonial conquests and ability to enslave others.